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href="#_02-【let和const】" aria-hidden="true">#</a></h1><h2 id="_1-let-命令" tabindex="-1">1.let 命令 <a class="header-anchor" href="#_1-let-命令" aria-hidden="true">#</a></h2><h3 id="_1-1-基本用法" tabindex="-1">1.1 基本用法 <a class="header-anchor" href="#_1-1-基本用法" aria-hidden="true">#</a></h3><p>ES6 新增了<code>let</code>命令，用来声明变量。它的用法类似于<code>var</code>，但是所声明的变量，只在<code>let</code>命令所在的代码块内有效。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">10</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">var</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">b</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">a </span><span style="color:#676E95;font-style:italic;">// ReferenceError: a is not defined.</span></span>
<span class="line"><span style="color:#A6ACCD;">b </span><span style="color:#676E95;font-style:italic;">// 1</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码在代码块之中，分别用<code>let</code>和<code>var</code>声明了两个变量。然后在代码块之外调用这两个变量，结果<code>let</code>声明的变量报错，<code>var</code>声明的变量返回了正确的值。这表明，<code>let</code>声明的变量只在它所在的代码块有效。</p><p><code>for</code>循环的计数器，就很合适使用<code>let</code>命令。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#A6ACCD;"> (</span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">&lt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">10</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i</span><span style="color:#89DDFF;">++</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// ...</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(i)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// ReferenceError: i is not defined</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，计数器<code>i</code>只在<code>for</code>循环体内有效，在循环体外引用就会报错。</p><p>下面的代码如果使用<code>var</code>，最后输出的是<code>10</code>。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> a </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> []</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#A6ACCD;"> (</span><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">&lt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">10</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i</span><span style="color:#89DDFF;">++</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;">[</span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;">] </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">};</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">a[</span><span style="color:#F78C6C;">6</span><span style="color:#A6ACCD;">]()</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> </span><span style="color:#676E95;font-style:italic;">// 10</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，变量<code>i</code>是<code>var</code>命令声明的，在全局范围内都有效，所以全局只有一个变量<code>i</code>。每一次循环，变量<code>i</code>的值都会发生改变，而循环内被赋给数组<code>a</code>的函数内部的<code>console.log(i)</code>，里面的<code>i</code>指向的就是全局的<code>i</code>。也就是说，所有数组<code>a</code>的成员里面的<code>i</code>，指向的都是同一个<code>i</code>，导致运行时输出的是最后一轮的<code>i</code>的值，也就是 10。</p><p>如果使用<code>let</code>，声明的变量仅在块级作用域内有效，最后输出的是 6。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> a </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> []</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#A6ACCD;"> (</span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">&lt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">10</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i</span><span style="color:#89DDFF;">++</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;">[</span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;">] </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">};</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">a[</span><span style="color:#F78C6C;">6</span><span style="color:#A6ACCD;">]()</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> </span><span style="color:#676E95;font-style:italic;">// 6</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，变量<code>i</code>是<code>let</code>声明的，当前的<code>i</code>只在本轮循环有效，所以每一次循环的<code>i</code>其实都是一个新的变量，所以最后输出的是<code>6</code>。你可能会问，如果每一轮循环的变量<code>i</code>都是重新声明的，那它怎么知道上一轮循环的值，从而计算出本轮循环的值？这是因为 JavaScript 引擎内部会记住上一轮循环的值，初始化本轮的变量<code>i</code>时，就在上一轮循环的基础上进行计算。</p><p>另外，<code>for</code>循环还有一个特别之处，就是设置循环变量的那部分是一个父作用域，而循环体内部是一个单独的子作用域。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#A6ACCD;"> (</span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">&lt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">3</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i</span><span style="color:#89DDFF;">++</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">abc</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// abc</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// abc</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// abc</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码正确运行，输出了 3 次<code>abc</code>。这表明函数内部的变量<code>i</code>与循环变量<code>i</code>不在同一个作用域，有各自单独的作用域（同一个作用域不可使用 <code>let</code> 重复声明同一个变量）。</p><h3 id="_1-2-不存在变量提升" tabindex="-1">1.2 不存在变量提升 <a class="header-anchor" href="#_1-2-不存在变量提升" aria-hidden="true">#</a></h3><p><code>var</code>命令会发生“变量提升”现象，即变量可以在声明之前使用，值为<code>undefined</code>。这种现象多多少少是有些奇怪的，按照一般的逻辑，变量应该在声明语句之后才可以使用。</p><p>为了纠正这种现象，<code>let</code>命令改变了语法行为，它所声明的变量一定要在声明后使用，否则报错。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// var 的情况</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(foo)</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> </span><span style="color:#676E95;font-style:italic;">// 输出undefined</span></span>
<span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> foo </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">2</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// let 的情况</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(bar)</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> </span><span style="color:#676E95;font-style:italic;">// 报错ReferenceError</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> bar </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">2</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，变量<code>foo</code>用<code>var</code>命令声明，会发生变量提升，即脚本开始运行时，变量<code>foo</code>已经存在了，但是没有值，所以会输出<code>undefined</code>。变量<code>bar</code>用<code>let</code>命令声明，不会发生变量提升。这表示在声明它之前，变量<code>bar</code>是不存在的，这时如果用到它，就会抛出一个错误。</p><h3 id="_1-3-暂时性死区" tabindex="-1">1.3 暂时性死区 <a class="header-anchor" href="#_1-3-暂时性死区" aria-hidden="true">#</a></h3><p>只要作用域内存在 let、const，它们所声明的变量或常量就自动 “绑定” 这个区域，不再受到外部作用域的影响。</p><div class="language-javascript line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> a </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">2</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">func</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;">        </span><span style="color:#676E95;font-style:italic;">// 报错</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#82AAFF;">func</span><span style="color:#A6ACCD;">()</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><div class="language-javascript line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> a </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">2</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">func</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;">        </span><span style="color:#676E95;font-style:italic;">// 2</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#82AAFF;">func</span><span style="color:#A6ACCD;">()</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p><strong>即：只要作用域内出现了同名的 let 或 const，那么就会去找这个量（向前找），如果找不到也不会跳去外部找，只会直接报错！</strong></p><blockquote><p>只要我们遵守 “<strong>先声明后使用</strong>”，那么其实就基本不会遇到变量提升及暂时性死区问题。</p></blockquote><h3 id="_1-4-不允许重复声明" tabindex="-1">1.4 不允许重复声明 <a class="header-anchor" href="#_1-4-不允许重复声明" aria-hidden="true">#</a></h3><p><code>let</code>不允许在相同作用域内，重复声明同一个变量。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 报错</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">func</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">10</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">var</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 报错</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">func</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">10</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>因此，不能在函数内部重新声明参数。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">func</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">arg</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">arg</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#82AAFF;">func</span><span style="color:#A6ACCD;">() </span><span style="color:#676E95;font-style:italic;">// 报错</span></span>
<span class="line"></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">func</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">arg</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">arg</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#82AAFF;">func</span><span style="color:#A6ACCD;">() </span><span style="color:#676E95;font-style:italic;">// 不报错</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><h2 id="_2-块级作用域" tabindex="-1">2.块级作用域 <a class="header-anchor" href="#_2-块级作用域" aria-hidden="true">#</a></h2><h3 id="_2-1-为什么需要块级作用域？" tabindex="-1">2.1 为什么需要块级作用域？ <a class="header-anchor" href="#_2-1-为什么需要块级作用域？" aria-hidden="true">#</a></h3><p>ES5 只有全局作用域和函数作用域，没有块级作用域，这带来很多不合理的场景。</p><p>第一种场景，内层变量可能会覆盖外层变量。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> tmp </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">new</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Date</span><span style="color:#A6ACCD;">()</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">tmp</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#F07178;"> (</span><span style="color:#FF9CAC;">false</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#C792EA;">var</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">tmp</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">hello world</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#82AAFF;">f</span><span style="color:#A6ACCD;">()</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> </span><span style="color:#676E95;font-style:italic;">// undefined</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码的原意是，<code>if</code>代码块的外部使用外层的<code>tmp</code>变量，内部使用内层的<code>tmp</code>变量。但是，函数<code>f</code>执行后，输出结果为<code>undefined</code>，原因在于变量提升，导致内层的<code>tmp</code>变量覆盖了外层的<code>tmp</code>变量。</p><p>第二种场景，用来计数的循环变量泄露为全局变量。</p><div class="language-"><pre><code>var s = &#39;hello&#39;;

for (var i = 0; i &lt; s.length; i++) {
  console.log(s[i]);
}

console.log(i); // 5
</code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，变量<code>i</code>只用来控制循环，但是循环结束后，它并没有消失，泄露成了全局变量。</p><h3 id="_2-2-es6-的块级作用域" tabindex="-1">2.2 ES6 的块级作用域 <a class="header-anchor" href="#_2-2-es6-的块级作用域" aria-hidden="true">#</a></h3><p><code>let</code>实际上为 JavaScript 新增了块级作用域。</p><ul><li><p>作用域链：内层作用域 ——&gt; 外层作用域 ——&gt; 全局作用域</p></li><li><p>块级作用域：除了对象 <code>{}</code>，函数 <code>{}</code>（函数作用域）之外的一切 <code>{}</code> 都属于块级作用域。</p></li></ul><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">f1</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">n</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">5</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#F07178;"> (</span><span style="color:#FF9CAC;">true</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">n</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">10</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">n</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#676E95;font-style:italic;">// 5</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面的函数有两个代码块，都声明了变量<code>n</code>，运行后输出 5。这表示外层代码块不受内层代码块的影响。如果两次都使用<code>var</code>定义变量<code>n</code>，最后输出的值才是 10。</p><p>ES6 允许块级作用域的任意嵌套。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#89DDFF;">{{{{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">{</span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">insane</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">Hello World</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">insane</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#676E95;font-style:italic;">// 报错</span></span>
<span class="line"><span style="color:#89DDFF;">}}}};</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码使用了一个五层的块级作用域，每一层都是一个单独的作用域。第四层作用域无法读取第五层作用域的内部变量。</p><p>内层作用域可以定义外层作用域的同名变量。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#89DDFF;">{{{{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">insane</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">Hello World</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">{</span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">insane</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">Hello World</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}}}};</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>块级作用域的出现，实际上使得获得广泛应用的匿名立即执行函数表达式（匿名 IIFE）不再必要了。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// IIFE 写法</span></span>
<span class="line"><span style="color:#A6ACCD;">(</span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">var</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">tmp</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">...;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">...</span></span>
<span class="line"><span style="color:#89DDFF;">}</span><span style="color:#A6ACCD;">())</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 块级作用域写法</span></span>
<span class="line"><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">tmp</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">...;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">...</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><h3 id="_2-3-块级作用域与函数声明" tabindex="-1">2.3 块级作用域与函数声明 <a class="header-anchor" href="#_2-3-块级作用域与函数声明" aria-hidden="true">#</a></h3><p>函数能不能在块级作用域之中声明？这是一个相当令人混淆的问题。</p><p>ES5 规定，函数只能在顶层作用域和函数作用域之中声明，不能在块级作用域声明。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 情况一</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#A6ACCD;"> (</span><span style="color:#FF9CAC;">true</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 情况二</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">try</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;font-style:italic;">catch</span><span style="color:#A6ACCD;">(e) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// ...</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面两种函数声明，根据 ES5 的规定都是非法的。</p><p>但是，浏览器没有遵守这个规定，为了兼容以前的旧代码，还是支持在块级作用域之中声明函数，因此上面两种情况实际都能运行，不会报错。</p><p>ES6 引入了块级作用域，明确允许在块级作用域之中声明函数。ES6 规定，块级作用域之中，函数声明语句的行为类似于<code>let</code>，在块级作用域之外不可引用。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">I am outside!</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">(</span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#F07178;"> (</span><span style="color:#FF9CAC;">false</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">    </span><span style="color:#676E95;font-style:italic;">// 重复声明一次函数f</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">I am inside!</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#82AAFF;">f</span><span style="color:#F07178;">()</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span><span style="color:#A6ACCD;">())</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码在 ES5 中运行，会得到“I am inside!”，因为在<code>if</code>内声明的函数<code>f</code>会被提升到函数头部，实际运行的代码如下。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// ES5 环境</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">I am outside!</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">(</span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">I am inside!</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#F07178;"> (</span><span style="color:#FF9CAC;">false</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#82AAFF;">f</span><span style="color:#F07178;">()</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span><span style="color:#A6ACCD;">())</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>ES6 就完全不一样了，理论上会得到“I am outside!”。因为块级作用域内声明的函数类似于<code>let</code>，对作用域之外没有影响。但是，如果你真的在 ES6 浏览器中运行一下上面的代码，是会报错的，这是为什么呢？</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 浏览器的 ES6 环境</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">I am outside!</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">(</span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#F07178;"> (</span><span style="color:#FF9CAC;">false</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">    </span><span style="color:#676E95;font-style:italic;">// 重复声明一次函数f</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">I am inside!</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#82AAFF;">f</span><span style="color:#F07178;">()</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span><span style="color:#A6ACCD;">())</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// Uncaught TypeError: f is not a function</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面的代码在 ES6 浏览器中，都会报错。</p><p>原来，如果改变了块级作用域内声明的函数的处理规则，显然会对老代码产生很大影响。</p><ul><li>允许在块级作用域内声明函数。</li><li>函数声明类似于<code>var</code>，即会提升到全局作用域或函数作用域的头部。</li><li>同时，函数声明还会提升到所在的块级作用域的头部。</li></ul><p>注意，上面三条规则只对 ES6 的浏览器实现有效，其他环境的实现不用遵守，还是将块级作用域的函数声明当作<code>let</code>处理。</p><p>根据这三条规则，浏览器的 ES6 环境中，块级作用域内声明的函数，行为类似于<code>var</code>声明的变量。上面的例子实际运行的代码如下。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 浏览器的 ES6 环境</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">I am outside!</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">(</span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">var</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">f</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">undefined;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#F07178;"> (</span><span style="color:#FF9CAC;">false</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">I am inside!</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#82AAFF;">f</span><span style="color:#F07178;">()</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span><span style="color:#A6ACCD;">())</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// Uncaught TypeError: f is not a function</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>考虑到环境导致的行为差异太大，应该避免在块级作用域内声明函数。如果确实需要，也应该写成函数表达式，而不是函数声明语句。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 块级作用域内部的函数声明语句，建议不要使用</span></span>
<span class="line"><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">secret</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 块级作用域内部，优先使用函数表达式</span></span>
<span class="line"><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">a</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">secret</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">f</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">};</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>另外，还有一个需要注意的地方。ES6 的块级作用域必须有大括号，如果没有大括号，JavaScript 引擎就认为不存在块级作用域。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 第一种写法，报错</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#A6ACCD;"> (</span><span style="color:#FF9CAC;">true</span><span style="color:#A6ACCD;">) </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> x </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 第二种写法，不报错</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#A6ACCD;"> (</span><span style="color:#FF9CAC;">true</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，第一种写法没有大括号，所以不存在块级作用域，而<code>let</code>只能出现在当前作用域的顶层，所以报错。第二种写法有大括号，所以块级作用域成立。</p><p>函数声明也是如此，严格模式下，函数只能声明在当前作用域的顶层。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 不报错</span></span>
<span class="line"><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">use strict</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#A6ACCD;"> (</span><span style="color:#FF9CAC;">true</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">function</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 报错</span></span>
<span class="line"><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">use strict</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#A6ACCD;"> (</span><span style="color:#FF9CAC;">true</span><span style="color:#A6ACCD;">)</span></span>
<span class="line"><span style="color:#A6ACCD;">  </span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">f</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><h2 id="_3-const-命令" tabindex="-1">3.const 命令 <a class="header-anchor" href="#_3-const-命令" aria-hidden="true">#</a></h2><h3 id="_3-1-基本用法" tabindex="-1">3.1 基本用法 <a class="header-anchor" href="#_3-1-基本用法" aria-hidden="true">#</a></h3><p><code>const</code>声明一个只读的常量。一旦声明，常量的值就不能改变。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> PI </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">3.1415</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">PI </span><span style="color:#676E95;font-style:italic;">// 3.1415</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">PI </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">3</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// TypeError: Assignment to constant variable.</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码表明改变常量的值会报错。</p><p><code>const</code>声明的变量不得改变值，这意味着，<code>const</code>一旦声明变量，就必须立即初始化，不能留到以后赋值。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> foo</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// SyntaxError: Missing initializer in const declaration</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码表示，对于<code>const</code>来说，只声明不赋值，就会报错。</p><p><code>const</code>的作用域与<code>let</code>命令相同：只在声明所在的块级作用域内有效。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#A6ACCD;"> (</span><span style="color:#FF9CAC;">true</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">const</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">MAX</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">5</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">MAX </span><span style="color:#676E95;font-style:italic;">// Uncaught ReferenceError: MAX is not defined</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p><code>const</code>命令声明的常量也是不提升，同样存在暂时性死区，只能在声明的位置后面使用。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#A6ACCD;"> (</span><span style="color:#FF9CAC;">true</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">MAX</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#676E95;font-style:italic;">// ReferenceError</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">const</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">MAX</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">5</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码在常量<code>MAX</code>声明之前就调用，结果报错。</p><p><code>const</code>声明的常量，也与<code>let</code>一样不可重复声明。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> message </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">Hello!</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> age </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">25</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 以下两行都会报错</span></span>
<span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> message </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">Goodbye!</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> age </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">30</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><h3 id="_3-2-本质" tabindex="-1">3.2 本质 <a class="header-anchor" href="#_3-2-本质" aria-hidden="true">#</a></h3><p><code>const</code>实际上保证的，并不是变量的值不得改动，而是变量指向的那个内存地址所保存的数据不得改动。对于简单类型的数据（数值、字符串、布尔值），值就保存在变量指向的那个内存地址，因此等同于常量。但对于复合类型的数据（主要是对象和数组），变量指向的内存地址，保存的只是一个指向实际数据的指针，<code>const</code>只能保证这个指针是固定的（即总是指向另一个固定的地址），至于它指向的数据结构是不是可变的，就完全不能控制了。因此，将一个对象声明为常量必须非常小心。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> foo </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{};</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 为 foo 添加一个属性，可以成功</span></span>
<span class="line"><span style="color:#A6ACCD;">foo</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">prop </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">123</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">foo</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">prop </span><span style="color:#676E95;font-style:italic;">// 123</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 将 foo 指向另一个对象，就会报错</span></span>
<span class="line"><span style="color:#A6ACCD;">foo </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{};</span><span style="color:#A6ACCD;"> </span><span style="color:#676E95;font-style:italic;">// TypeError: &quot;foo&quot; is read-only</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，常量<code>foo</code>储存的是一个地址，这个地址指向一个对象。不可变的只是这个地址，即不能把<code>foo</code>指向另一个地址，但对象本身是可变的，所以依然可以为其添加新属性。</p><p>下面是另一个例子。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> a </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> []</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">push</span><span style="color:#A6ACCD;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">Hello</span><span style="color:#89DDFF;">&#39;</span><span style="color:#A6ACCD;">)</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> </span><span style="color:#676E95;font-style:italic;">// 可执行</span></span>
<span class="line"><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">length </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;">    </span><span style="color:#676E95;font-style:italic;">// 可执行</span></span>
<span class="line"><span style="color:#A6ACCD;">a </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> [</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">Dave</span><span style="color:#89DDFF;">&#39;</span><span style="color:#A6ACCD;">]</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;">    </span><span style="color:#676E95;font-style:italic;">// 报错</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，常量<code>a</code>是一个数组，这个数组本身是可写的，但是如果将另一个数组赋值给<code>a</code>，就会报错。</p><h3 id="_3-3-window-对象的属性和方法（全局作用域中）" tabindex="-1">3.3 window 对象的属性和方法（全局作用域中） <a class="header-anchor" href="#_3-3-window-对象的属性和方法（全局作用域中）" aria-hidden="true">#</a></h3><p>全局作用域中，<code>var</code> 声明的变量，<code>function</code> 声明的函数，会自动变成 window 对象的属性或方法。</p><div class="language-javascript line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> age </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">18</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">add</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{}</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(window</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">age)</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;">            </span><span style="color:#676E95;font-style:italic;">// 18</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(window</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">add </span><span style="color:#89DDFF;">===</span><span style="color:#A6ACCD;"> add)</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;">     </span><span style="color:#676E95;font-style:italic;">// true</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><div class="language-javascript line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> age </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">18</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> add </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">function</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{}</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(window</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">age)</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;">            </span><span style="color:#676E95;font-style:italic;">// undefined</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(window</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">add </span><span style="color:#89DDFF;">===</span><span style="color:#A6ACCD;"> add)</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;">     </span><span style="color:#676E95;font-style:italic;">// false</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h3 id="_3-4-什么时候用-let，什么使用用-const" tabindex="-1">3.4 什么时候用 let，什么使用用 const <a class="header-anchor" href="#_3-4-什么时候用-let，什么使用用-const" aria-hidden="true">#</a></h3><p>原则：如果不知道用什么的时候，就用 const</p><p>原因：如果应该是常量，那么刚好符合需求。如果应该是变量，那么后来报错时，再来改为变量也为时不晚。同时，一开始就设置为常量还会避免真的需要为常量时，该值在后来被意外修改的情况。</p><h2 id="_4-let和const总结" tabindex="-1">4.let和const总结 <a class="header-anchor" href="#_4-let和const总结" aria-hidden="true">#</a></h2><ol><li>let 声明的变量会产生块作用域，var 不会产生块作用域</li><li>const 声明的常量也会产生块作用域</li><li>不同代码块之间的变量无法互相访问</li><li>注意: 对象属性修改和数组元素变化不会出发 const 错误 （数组和对象存的是引用地址）</li><li>应用场景：声明对象类型使用 const，非对象类型声明选择 let</li><li>cosnt声明必须赋初始值，标识符一般为大写，值不允许修改。</li></ol><h2 id="_5-顶层对象的属性" tabindex="-1">5.顶层对象的属性 <a class="header-anchor" href="#_5-顶层对象的属性" aria-hidden="true">#</a></h2><p>顶层对象，在浏览器环境指的是<code>window</code>对象，在 Node 指的是<code>global</code>对象。ES5 之中，顶层对象的属性与全局变量是等价的。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">window</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">a </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">a </span><span style="color:#676E95;font-style:italic;">// 1</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">a </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">2</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">window</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">a </span><span style="color:#676E95;font-style:italic;">// 2</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，顶层对象的属性赋值与全局变量的赋值，是同一件事。</p><p>顶层对象的属性与全局变量挂钩，被认为是 JavaScript 语言最大的设计败笔之一。这样的设计带来了几个很大的问题，首先是没法在编译时就报出变量未声明的错误，只有运行时才能知道（因为全局变量可能是顶层对象的属性创造的，而属性的创造是动态的）；其次，程序员很容易不知不觉地就创建了全局变量（比如打字出错）；最后，顶层对象的属性是到处可以读写的，这非常不利于模块化编程。另一方面，<code>window</code>对象有实体含义，指的是浏览器的窗口对象，顶层对象是一个有实体含义的对象，也是不合适的。</p><p>ES6 为了改变这一点，一方面规定，为了保持兼容性，<code>var</code>命令和<code>function</code>命令声明的全局变量，依旧是顶层对象的属性；另一方面规定，<code>let</code>命令、<code>const</code>命令、<code>class</code>命令声明的全局变量，不属于顶层对象的属性。也就是说，从 ES6 开始，全局变量将逐步与顶层对象的属性脱钩。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> a </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 如果在 Node 的 REPL 环境，可以写成 global.a</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 或者采用通用方法，写成 this.a</span></span>
<span class="line"><span style="color:#A6ACCD;">window</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">a </span><span style="color:#676E95;font-style:italic;">// 1</span></span>
<span class="line"></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> b </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">window</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">b </span><span style="color:#676E95;font-style:italic;">// undefined</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，全局变量<code>a</code>由<code>var</code>命令声明，所以它是顶层对象的属性；全局变量<code>b</code>由<code>let</code>命令声明，所以它不是顶层对象的属性，返回<code>undefined</code>。</p></div></div><!----></main><!--[--><!--]--><!----></div></div></div></div><div class="visually-hidden" aria-live="polite" data-v-23306c50>02 【let和const】 has loaded</div></div></div>
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